linux-kernel - RE: [PATCH 1/1] iommu/vt-d: Omit devTLB invalidation requests when TES=0

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Date:   Tue, 14 Nov 2023 04:45:40 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        "Will Deacon" <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        "Jason Gunthorpe" <jgg@...pe.ca>
CC:     "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/1] iommu/vt-d: Omit devTLB invalidation requests when
 TES=0

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Tuesday, November 14, 2023 11:14 AM
> 
> On 11/14/23 11:14 AM, Tian, Kevin wrote:
> >> From: Lu Baolu<baolu.lu@...ux.intel.com>
> >> Sent: Tuesday, November 14, 2023 9:11 AM
> >>
> >> The latest VT-d spec indicates that when remapping hardware is disabled
> >> (TES=0 in Global Status Register), upstream ATS Invalidation Completion
> >> requests are treated as UR (Unsupported Request).
> >>
> >> Consequently, the spec recommends in section 4.3 Handling of Device-TLB
> >> Invalidations that software refrain from submitting any Device-TLB
> >> invalidation requests when address remapping hardware is disabled.
> >>
> >> Verify address remapping hardware is enabled prior to submitting Device-
> >> TLB invalidation requests.
> >>
> >> Fixes: 792fb43ce2c9 ("iommu/vt-d: Enable Intel IOMMU scalable mode by
> >> default")
> >> Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
> >> ---
> >>   drivers/iommu/intel/dmar.c | 18 ++++++++++++++++++
> >>   1 file changed, 18 insertions(+)
> >>
> >> diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c
> >> index a3414afe11b0..23cb80d62a9a 100644
> >> --- a/drivers/iommu/intel/dmar.c
> >> +++ b/drivers/iommu/intel/dmar.c
> >> @@ -1522,6 +1522,15 @@ void qi_flush_dev_iotlb(struct intel_iommu
> >> *iommu, u16 sid, u16 pfsid,
> >>   {
> >>   	struct qi_desc desc;
> >>
> >> +	/*
> >> +	 * VT-d spec, section 4.3:
> >> +	 *
> >> +	 * Software is recommended to not submit any Device-TLB
> >> invalidation
> >> +	 * requests while address remapping hardware is disabled.
> >> +	 */
> >> +	if (!(iommu->gcmd & DMA_GCMD_TE))
> >> +		return;
> >> +
> > Is it a valid case to see such request when the iommu is disabled?
> > If not then let's add a WARN.
> 
> There might be valid cases. The VT-d translation is turned on after all
> devices get probed.
> 

but I didn't get why there will be actual mapping changes before 
vtd translation is enabled...